Cholesteric liquid crystal displays are characterized by the fact that the pictures stay on the display even if the driving voltage is disconnected. The bistability and multistability also ensure a completely flicker-free static display and have the possibility of infinite multiplexing to create giant displays and/or ultra-high resolution displays. In cholesteric liquid crystals, the molecules are oriented in helices with a periodicity characteristic of material. In the planar state, the axis of this helix is perpendicular to the display plane. Light with a wavelength matching the pitch of the helix is reflected and the display appears bright. If an AC-voltage is applied, the structure of the liquid crystals changes from planar to focal conic texture. The focal-conic state is predominately characterized by its highly diffused light scattering appearance caused by a distribution of small, birefringence domains, at the boundary between those domains the refractive index is abrupt changed. This texture has no single optic axis. The focal-conic texture is typically milky-white (i.e., white light scattering). Both planar texture and focal-conic texture can coexist in the same panel or entity. This is a very important property for display applications, whereby the gray scale can be realized.
Current cholesterics displays are utilizing xe2x80x9cBragg reflectionxe2x80x9d, one of the intrinsic properties of cholesterics. In Bragg reflection only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer, which generates a monochrome color display. The remaining spectrum of the incoming light, however, including the 50% opposite handedness circular polarized and out-off Bragg reflection wave band, will pass through the display and be absorbed by the black coating material on the back surface of the display to assure the contrast ratio. The overall light utilization efficiency is rather low and it is not qualified in some applications, such as billboard at normal ambient lighting condition. The Bragg type reflection gives an impression that monochrome display is one of distinctive properties of the ChLCD.
In many applications the human eyes are friendlier with full color spectrum, i.e., white color information written on the dark background. With the development of the flat panel display more and more displays with neutral color is come into being, such as black-and-white STN display and AMTFT display, etc. Unfortunately, both of these approaches involve major disadvantages and limitations. The AMTFT displays are not true zero field image storage systems, as they require constant power input for image refreshing. The STN displays do not possess inherent gray scale capability as a result of the extreme steepness of the electro-optical response curve of the display. To realize a gray scale the resolution has to be reduced by using, for example, four pixels instead of one per area. Anywhere from one to four pixels are activated at a particular time to provide the gray scale effect. The AMTFT devices use semiconductors to provide memory effects and involve use of expensive, ultra high resistance liquid crystal materials to minimize RC losses. Additionally, these displays are both difficult and costly to produce and they are, at present, limited to relatively small size displays. The cholesteric display has many advantages over the STN and AMTFT display with its zero field memory effect, hemispheric viewing angle, gray scale capability and other optical performances but it needs obviously to come up with black-and-white solution in order to keep its superiority.
U.S. Pat. No. 5,796,454 introduces a black-and-white back-lit ChLC display. It includes controllable ChLC structure, a first circular polarizer laminating to a first substrate of cell has the same circular polarity as the liquid crystals, a second circular polarizer laminating to a second substrate of the cell has a circular polarity opposite to the liquid crystals, and a light source. The black-and-white back-lit display is preferably illuminated by a light source that produces natural xe2x80x9cwhitexe2x80x9d light. Thus, when the display is illuminated by incident light, the circular polarizer transmits the 50% component of the incident light that is right-circularly polarized. When the ChLC is in an xe2x80x9conxe2x80x9d state, the light reflected by the ChLC is that portion of the incident light having wavelengths within the intrinsic spectral bandwidth, and the same handedness; The light that is transmitted through the ChLC is the complement of the intrinsic color of ChLC. The transmitted light has right-circular polarization, however, is thus blocked by left-circular polarizer. Therefore, the observer will perceive that region of the display to be substantially black. When the display is in an xe2x80x9coffxe2x80x9d state, the light transmitted through the polarizer is optically scattered by the ChLC. The portion of the incident light that is forward scattered is emitted from the controllable ChLC structure as depolarized light. The left-circularly polarized portion of the forward-scattered light is transmitted through the left-circular polarizer, thus, is perceived by an observer. The black-and-white display, in xe2x80x9c454xe2x80x9d patent, is generated by back-lit component and the ambient light is nothing but xe2x80x9cnoisexe2x80x9d.
It is well known that ChLCD can be used perfectly for daytime information display for its bright reflection to the front environmental lighting. Nowadays available cholesterics display is generally working in reflection mode and the black painting absorbs incoming light both opposite the handedness and out-off the selective reflection wave band. It is the black coating material that completely rolls out the possibilities of adopting the back-lit approach.
In some applications it is needed to work after dark. Take a cell phone for example, a customer need make phone call any time including nighttime and dark environment (travelling in the car). This makes ChLCD an artificial lighting system necessary. It is convenient to use front light arrangement for some types of displays such as gas pump and billboards. In the case of hand-held type of display however, the compact and ergonomic designing requires a back-lit structure.
It is the primary intention for this invention to realize the full spectrum of reflection (white color in optical xe2x80x9conxe2x80x9d state) while maintaining the cholesterics display""s superiority such as high ambient environment contrast ratio, hemispheric viewing angle, zero-field long time memory and so on.
It is the other intention for this invention to create the dark state for the optical xe2x80x9coffxe2x80x9d state so as to achieve black-and-white display, which is the foundation of the achromatic display and the full color display.
It is the other intention of this invention to render the display dual working function, i.e., during the day or bright ambient light the display works as front-lit mode and during the dark environment, back-it mode. Compared with the prior art ChLC displays the skill of the art endows the display with more user friendly and better viewing quality.
The invention is based upon the principles described as following:
Optical xe2x80x9cOnxe2x80x9d State
First, the cholesteric material in planar texture reflects the light component with the same handedness as ChLC and a narrow bandwidth determined by its helical pitch and the optical birefringence (prior art Bragg reflection). Secondly the remaining light component out of the selective bandwidth passes the cell again, is reflected by a metal material without changing the polarization state, i.e., handedness (there is no half wave phase loss) and this component reentry the ChLC cell from back side without attenuation. The two components, one reflected by Bragg reflection with a center wavelength xcex0, and the other reflected by the metal surface, are compensatory each other and will meet together and emanate toward the viewer as full gamut of visible light. When the ChLCD is tuned in invisible Bragg reflection wavelength, for example in infrared wavelength, a full spectrum of visible light will be reflected by the metal surface, thus the viewer still perceives full spectrum white color.
Optical xe2x80x9cOffxe2x80x9d State
The incoming light reaches a circular polarizer with the same handedness of the ChLC and is cut more than 50%. The rest gets to the ChLC cell with focal conic texture and is depolarized by the scattering effect of the LC material. The light passes linear polarizer being cut more than 50%, then is reflected by the metal surface and further passes through either a circular polarizer or a color filter or both of them, located between the ChLC cell and the metal reflector. The remaining light passes through the ChLC cell again is depolarized by the focal conic scattered texture then cut out more than half of it by the front circular polarizer, finally, only small portion of total light has a chance to reach to the viewer. As a result, the special designed optical path, polarized-depolarized-polarized-depolarized-polarized, create a new optical dark state of cholesteric liquid crystal display. The term xe2x80x9cpolarizedxe2x80x9d means some of the light being absorbed and others becoming polarized light. And the term xe2x80x9cdepolarizedxe2x80x9d means the polarized light being neutralized by the scattering domain of LC and ready to be further cut-off The optical xe2x80x9coffxe2x80x9d state produced in this way is brand new in liquid crystal display history.
In the prior arts the liquid crystal""s scattering depolarization effect is used for optical xe2x80x9conxe2x80x9d state, i.e., DTN (depolarized twist nematic), DPDLC (depolarized polymer dispersed liquid crystal), and transflective mode cholesteric liquid crystal display. The principle of those displays is based upon the sandwiched structure of LC cell and two layers of polarizers. When the LC cell is in the scattering state the polarized light will be depolarized and the partial light will pass the second polarizer so the display takes on the bright state or optical xe2x80x9conxe2x80x9d state. When the LCD cell is in the field induced homeotropic phase or planar state the incoming light will cut completely by both the front and back polarizers so that the display takes on optical xe2x80x9coffxe2x80x9d state. The problem for this arrangement is that the total transparency is rather low because 75% incoming light will be cut by the combination of the polarizers and only less than 25% percent light could pass the LCD cell. This invention, however, for the first time, introduces a novel approach of applying the depolarization effect to the optical xe2x80x9coffxe2x80x9d state. It is described that the incoming light will be absorbed, depolarized, again absorbed, depolarized, and absorbed and finally the display almost becomes dark or black. The depolarizing efficiency of the scattering structure is entirely dependent on the reflective index of LC material and the thickness of the LC cell. The discovery in the skill of the art will create a new display dark state by using liquid crystal scattering effect.
The addition of the color filter or circular polarizer between the cell and metal reflector has no effect in the optical xe2x80x9conxe2x80x9d state. The color filter leaves a path window to the remaining same handedness polarized light out of the intrinsic Bragg reflection wavelength and theoretically no light attenuation takes place. The circular polarizer faces to the metal reflector also creates a path window to the circular polarizing light and again there is no discernible attenuation to the passing light.
The full spectrum white display mode allows the LCD designer to use the longest visible wavelength such as red, so that the lowest driving voltage can be obtained and darkness of the optical xe2x80x9coffxe2x80x9d state can also be enhanced. In this case, the compensatory color is green-blue. If a green-blue color is chosen as the color filter""s tint the display will takes on very dark green-blue xe2x80x9coffxe2x80x9d state. Further more if the display uses the combination of both band-pass filter and circular polarizer the optical xe2x80x9coffxe2x80x9d state will be very dark.
The contrast ratio is enhanced because the brightness of the display is increased while the darkness is kept the same level as the prior art display. This property facilitates the display applying in the relatively dark environment because of high efficiency of light utilization. The viewing angle is kept almost hemispheric scope without noticeable color shift.
The full spectrum of reflective cholesteric display can be realized both in visible wavelength and in invisible wavelength, for example, infrared wave band. The optical scattering effect in the infrared wavelength is the same as in visible wavelength, which is dependent on the refractive index and the pitch of the liquid crystals so that the display obtains the same optical dark state. Optical xe2x80x9conxe2x80x9d state on the other hand is the fill gamut of visible light reflected by a metal reflector. A cholesteric display that works in infrared wavelength will have very fast response time and low driving voltage for the reason of lower viscosity and longer helical pitch. In the infrared wavelength, linear polarizer with the condition of optimal alignment angle can replace circular polarizer.